"More than a week after the shooting, investigators were still finding bullets under doors and in carpets and walls in the school as they tried to match the casings to the magazines.”
What is difficult about matching casings to magazines if they were all shot with the same rifle?
Hollowpoint bullets would have shattered if they hit the walls or floors so it would actually be rather difficult to account for every fragment to ensure that another weapon wasn't used. Investigators like these don't like any questions. The ones I know are very OCD.
He used about 10 different magazines. Apparently they are trying to trace them back to which magazine each came from. No wonder this investigation is taking so long.